1. Field of the Invention
The present invention relates to pressure sensors used in the measurement of pressure in fluid mediums, which mediums may be either liquid or gas.
2. Prior Art
Pressure sensors known in the prior art generally teach a pressure or force responsive diaphragm forming one plate or electrode of a capacitor. This electrode or capacitor plate is subject to deformation, the extent of which is compared to a second electrode means or capacitive plate that is not displaced. As the deformation varies the distance between such capacitor plates, an electrical or electrically translatable signal is produced which can be calibrated to relate the deformation to the deforming force. The production of such capacitor sensors or transducers is well known in the art.
An inherent part of the capacitive sensor structures available today is a ceramic diaphragm operator. However, the deformation of any diaphragm operator results from the differential force acting on both sides of such a diaphragm. This differential force is then translated into an electrical analog signal through an electrode means or capacitive plate sensor which in fact measures the deformation or displacement of such diaphragm operator. These analog signals are thereafter calibrated or related to the measured or sensed parameter. Such a variable capacitance sensor is illustrated in U.S. Pat. No. 3,859,575 (Lee et al) wherein a rod and plate are provided to react to a force and apply it to capacitive or separated electrode plates calibrated to measure the force applied to the rod. More specifically, the embodiment taught at FIG. 4 provides a force applied to rod 90 to increase the separation between electrode plates 5 and 23. Diaphragm or plate 96 is primarily provided to ensure centering of rod 90 and may be provided with pressure relieving holes or apertures. Thus, the forces communicated to the electrodes for measurement are provided through rod member 90. Alternatively, pressure forces may be communicated to rod 9 as in FIG. 1 through chamber 31 which forces act on the lower surface of the diaphragm. However, such force is measured as a difference between the pressure on either side of surface 5.
A means for indicating pressure in subterranean formations is taught in U.S. Pat. No. 4,125,027 (Clark). This patent discloses a variable capacitance sensor responsive to changes in ambient pressure, not to a differential pressure across an orifice. Further, it uses an arm extending from its diaphragm operator as a centering means to maintain location of its electrode or stator 24. However, there is no means provided to measure a differential pressure across an orifice nor is there any means disclosed to provide such measurement in a remote setting to protect, or provide a protective environment for, the electrodes associated with this sensor. U.S. Pat. No. 4,382,377 (Kleinschmidt et al.) teaches a piezoelectric pressure sensor for detecting knock and ping. This sensor is designed to be secured in a cylinder head for an internal combustion engine with the membrane diaphragm located within a cylinder. The forces being measured are provided at membrane 15. Again there is no provision in the structure to provide a differential pressure across an orifice in a flow passage. The reference pressure against which the sensed pressure is compared may be atmosphere or a vacuum, but it is not a pressure drop across an orifice.
A differential pressure transducer is taught in U.S. Pat. No. 4,382,385 (Paros). This transducer includes an air tight enclosure with a pair of pressure ports coupled to opposite sides of a pressure-sensing diaphragm or bellows. The force generated by the pressure differential is coupled to a stress-sensitive resonator either directly or through a force-transmitting structure. This structure teaches the use of bellows operators in cooperation with a resonator member or resonant sensitive member to provide a measured signal. In U.S. Pat. No. 4,089,036 (Geronime) a capacitive type load cell is disclosed having a diaphragm member mounted to a support for movement relative thereto. However, there is no indication of communication of a pressure or differential pressure across the diaphragm face to provide the force for moving such diaphragm. Further, the relationship of the diaphragm and support button is provided to reduce radial bending stresses in the diaphragm during loading, which implies that all loading is provided external to the electronic structure. Therefore, Geronime '036 recognizes the need to provide protective environments for electronic components.
The objective of the above devices is to provide a variable capacitance type signal to measure applied force. This measurement is proportional to, or a function of, a change in distance between capacitor plates. Some of the references identified above recognized the problem associated with the introduction of electronic components into harsh environments, but did not propose effective solutions. In the case of Kleinschmidt et al. '377, the sensor has been partially encapsulated with an expensive electronic structure to overcome the introduction of the device into a harsh environment. A similar device for force measurement is taught by Everett/Charles Marketing Services, Inc. Their model FT655 includes a force transducer in contact with a mechanical arm to move a pressure transducer diaphragm. This device is advertised specifically for use in an environment exposed to compression forces. The present invention provides a means for measurement of a differential pressure across an orifice in a harsh environment which may include fluids at elevated temperatures and entrained particulates. It utilizes a pressure sensor, particularly a capacitive pressure sensor, without exposing the electronic circuitry thereof to either heat, corrosion or dielectric degradation. Therefore, the pressure drop across an orifice is continuously provided, and as the orifice is of a known size it provides a means to measure flow rate.